pack-bitmaps: plug memory leak, fix allocation size for recent_bitmaps

pack-bitmaps: plug memory leak, fix allocation size for recent_bitmaps

Use an automatic variable for recent_bitmaps, an array of pointers.
This way we don't allocate too much and don't have to free the memory
at the end.  The old code over-allocated because it reserved enough
memory to store all of the structs it is only pointing to and never
freed it.  160 64-bit pointers take up 1280 bytes, which is not too
much to be placed on the stack.

MAX_XOR_OFFSET is turned into a preprocessor constant to make it
constant enough for use in an non-variable array declaration.

Noticed-by: Stefan Beller <stefanbeller@gmail.com>
Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>